This invention relates to a vacuum evaporation coil, and more particularly relates to a composite vaccum evaporation coil of a single wire base coil with a short overwind coil.
Vaccum evaporation is a widely practiced method of producing thin metal layers. For example, aluminum reflective layers are often produced on the inside surface of lamp and cathode ray tube envelopes by flash evaporating a slug of solid aluminum inside the envelope while in a vacuum environment. The slug is generally in the shape of a crimped ribbon or cylinder, and is inserted into the windings of a heater coil, which windings hold the slug in place. Evaporation is carried out by first heating the slug until the slug material melts and clings to the turns of the coil, and then further heating the molten slug material to cause rapid (flash) evaporation. The vapor then forms a uniform deposit on the cooler substrate, that is, the inner surface of the glass envelope of the lamp or the cathode ray tube.
The major performance criteria for such vacuum evaporation coils are its evaporating efficiency and its operating life. Evaporating efficiency of a coil is governed primarily by the evaporating surface area and by the ability to hold the molten slug material in place until evaporation can occur.
The operating life of a coil is generally governed by its ability to resist the corrosive attack of the evaporating material, and is thus somewhat dependent upon the surface area of the coil as well as its corrosion-resistant properties.
It is known that the addition of an overwind coil to a single wire base coil can dramatically increase both the efficiency and the operating life of the coil, by providing a large evaporating surface area separate from the base coil. Since most of the molten slug material is confined to the overwind coil, its material can be chosen for its wetting properties, while the base coil material can be chosen primarily for its resistive heating and corrosion resistant characteristics.
In the past, however, use of an overwind coil with a single wire base coil has resulted in a decreased electrical resistance of the composite coil, thereby requiring a higher electrical current to heat the slug sufficiently to cause its evaporation. Reducing the diameter of the base coil wire in order to compensate for this decreased electrical resistance results in decreased corrosion resistance, and thus has a deleterious effect upon the operating life of the coil.
Accordingly, it is an object of the invention to provide a composite vacuum evaporating coil in which a base coil is provided with an overwind coil in order to increase the evaporating efficiency and the operating life of the coil, without decreasing the electrical resistance of the coil, and without decreasing the diameter of the base coil wire.